Drop down menu filter

I would like to have the ability to apply a filter to the drop-down menu so that users can see all of the options, but those matching a filter would be grayed out and inactive. In my use case, I have a directory for all of the schools in the state. Each school has an admin who can make changes to the data for their school. If that person leaves the school (retires, moves, etc.), the person who is put in charge of the school doesn’t automatically have admin privileges to that school’s data. I have to manually give them those rights. Even though I explain that in the app and in the documentation, they often just create a new school, and I have duplicates.

To fix that, I’ve applied a filter to the drop-down that doesn’t show any school that has an entry in the table. They are then confused about why. If it was there in gray, they would at least know the school exists. Having a pop-up to explain would be even better.